FIG. 1 is a schematic configuration diagram of an atomic absorption spectrophotometer 10 using a double beam optical system. The atomic absorption spectrophotometer 10 is mainly composed of: a hollow cathode (HC) lamp 11 which is a light source for emitting a light having a bright-line spectrum; a deuterium (D2) lamp 12 which is a light source for emitting a light having a continuous spectrum; a half mirror 13 for dividing the light emitted from each lamp into two directions; an atomization unit 18 for atomizing a sample; and a sector mirror 21 for sending each light divided by the half mirror 13 to the same optical path; a spectroscope 30 for separating the light sent from the sector mirror 21; a detector 36 for detecting the intensity of the separated light; and a controller 40 for performing a transmission of a variety of signals and data processing. For the details, refer to Patent Document 1.
FIG. 2 is a front view of the sector mirror 21. The sector mirror 21 is composed of two sector-shaped mirrors 211, each having a central angle of 90°, symmetrically-placed with respect to a shaft 212. A motor 22 is connected to the shaft 212, and the sector mirror 21 is rotated on the shaft 212 by the motor 22. On the rotational course of the mirror 211, a photo interrupter 213 is placed in such a manner as to sandwich the course. The photo interrupter 213 generates detection signals indicating whether the mirror 211 exists at the position, and the detection signals are transmitted to the controller 40.
The light from the light source which reaches the sector mirror 21 has a definite diameter on the sector mirror 21. As shown in FIG. 2, the photo interrupter 213 is placed at the symmetrical position, with respect to the shaft 212, to an area A which corresponds to the cross-section of the light. In this example, it is adjusted that the edges of the mirror 211 reach the area A and the photo interrupter 213 at approximately the same moment. When the edge of the mirror 211 reaches the area A, the state of the detection signal generated from the photo interrupter 213 is changed.
In the atomic absorption spectrophotometer 10, the lights emitted from the HC lamp 11 and the D2 lamp 12 are divided into two directions by the half mirror 13. One of the divided light (sample light Ls) passes through the atomization unit 18, and the other light (reference light Lr) passes through a space out of the atomization unit 18 and then reaches the same area A in the rotating sector mirror 21. In the case where the mirror 211 does not exist at the area A, the sample light Ls passes through the area, and in the case where the mirror 211 exists at the area A, the reference light Lr is reflected there and enters the spectroscope 30. Accordingly, the sample light Ls and the reference light Lr alternately enter the spectroscope 30. The light which has entered the spectroscope 30 is separated and a light having a specific wavelength enters the detector 36. In the detector 36, a detection signal corresponding to the intensity of the entered light is generated, and the detection signal is transmitted to the controller 40. The detection signal received by the controller 40 is analog-to-digital (A/D) converted at predetermined sampling intervals, and the sampling data is recorded in a data memory.
In the atomic absorption spectrophotometer 10 as just described, the absorbance of a sample is obtained as follows. First, an explanation is made for the case where only the HC lamp 11 is used as the light source. (Hereinafter, this case will be called a Non-BGC-Double measurement mode.) FIG. 3 illustrates a blinking manner of the HC lamp 11 in this case. The photo interrupter 213 generates an H level signal while the sample light Ls passes through the sector mirror 21 (S period), and generates an L level signal while the reference light Lr is reflected by the sector mirror 21 (R period). The HC lamp 11 is ON for the second predetermined period Ta2, which has the starting point after the first predetermined time Ta1 has passed from the time point when the S period and the R period are changed to each other, i.e. from the changing point P when the detection signal of the photo interrupter 213 changes. During the other period, the HC lamp 11 remains OFF.
The absorbance AH in this case is defined by the following equations:AH=−log10 TH,TH=(HS′×HR0′)/(HR′×HS0′).
The absorbance of the sample can be obtained by substituting the values corresponding to the following symbols to these equations.
HS: The average of the sampling data obtained while the HC lamp is ON in the S period.
DKS: The average of the sampling data obtained while the HC lamp is OFF in the S period.
HR: The average of the sampling data obtained while the HC lamp is ON in the R period.
DKR: The average of the sampling data obtained while the HC lamp is OFF in the R period.HS′=HS−DKS HR′=HR−DKR 
HS0′: HS′ that has been previously obtained while there was no sample, i.e. while the atomization of the sample was not being performed.
HR0′: HR′ that has been previously obtained while there was no sample, i.e. while the atomization of the sample was not being performed.
Next, an explanation is made for the case where a background correction is performed using the D2 lamp 12 as well as the HC lamp 11 as a light source. (Hereinafter, this case will be called a BGC-D2-Double measurement mode.) FIG. 4 illustrates a blinking manner of the HC lamp 11 and the D2 lamp 12 in this case. The photo interrupter 213 generates, as in the aforementioned Non-BGC-Double measurement mode, an H level signal in the S period, and L level signal in the R period. The HC lamp 11 is ON for the fourth predetermined period Tb2, which has a starting point after the third predetermined period Tb1 has passed from the changing point P of the detection signal of the photo interrupter 213, and the HC lamp 11 remains OFF during the other period. The lighting period of the HC lamp 11 and that of the D2 lamp 12 are set not to coincide with each other.
The absorbance ABGC in this case is defined by the following equations:ABGC=−log10 TBGC,TBGC=(HS′×HR0′×DR′×DS0′)/(HR′×HS0′×DS′×DR0′).
The absorbance of the sample which is background-corrected by the D2 lamp 12 can be obtained by substituting, to these equations, the values corresponding to the symbols used for obtaining the absorbance in the Non-BGC-Double measurement mode and the values corresponding to the following symbols.
DS: The average of the sampling data obtained while the D2 lamp is ON in the S period.
DR: The average of the sampling data obtained while the D2 lamp is ON in the R period.DS′=DS−DKS DR′=DR−DKR 
DS0′: DS′ that has been previously obtained while there was no sample, i.e. while the atomization of the sample was not being performed.
DR0′: DR′ that has been previously obtained while there was no sample, i.e. while the atomization of the sample was not being performed.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-14631
In general, as the motor 22 for rotating the sector mirror 21, an alternating-current (AC) motor is used in many cases in order to simplify the configuration, moderate the cost, and reduce the rotational noise as much as possible. And the AC motor is often powered by a commercial AC source. In such a case, the rotational speed of the AC motor is proportional to the alternating frequency of the AC power source, and the rotational speed of the sector mirror is also proportional to the frequency of the AC power source. The lengths of the S period and the R period are inversely proportional to the rotational speed of the sector mirror 21. Therefore, the lengths of the S period and R period are inversely proportional to the frequency of the AC power source.
FIG. 5 illustrates a blinking manner of the HC lamp 11 in the Non-BGC-Double measurement mode in the cases where the frequency is 60 Hz (FIG. 5A) and 50 Hz (FIG. 5B). The lengths of the S period and the R period are shorter in the case where the frequency is 60 Hz due to the aforementioned reason. Since the HC lamp 11 is set, in each case, to be ON only during the second predetermined period Ta2 which starts at the time point after the predetermined period Ta1 has passed from the changing point P of the detection signal of the photo interrupter 213, the first predetermined period Ta1 and the second predetermined period Ta2 are set for the case of 60 Hz where the interval of blinking is shorter than that of the case of 50 Hz.
However, with such a setting, in the case of 50 Hz where the S period and R period are longer, i.e. in the case where the alternating frequency of the AC power source is lower, the number of blinks of the lamp per unit time decreases. In the meantime, the second predetermined period Ta2 which is the lighting time is set to be a constant value, and therefore in the case where the frequency is low, the total lighting time per unit time becomes shorter and the number of pieces of sampling data obtained while the light is ON decreases. This increases the statistical error of the sampling data, which deteriorates the lowest detection limit performance of elements. This problem also occurs in the BGC-D2-Double measurement mode.